The present invention relates to a semiconductor laser module for coupling light emitted from a semiconductor laser light source to an optical fiber, and a light scanning device and an image forming apparatus using the same.
A semiconductor laser module by which light emitted from a semiconductor laser light source is efficiently led into a single mode optical fiber has been heretofore widely used chiefly in the field of optical communication. The single mode optical fiber has the advantage of small core diameter and low transmission loss.
FIG. 14 is a view showing a structure of a system for coupling a semiconductor laser light source and an optical fiber to each other according to the background art. As shown in FIG. 14, laser light emitted from a semiconductor laser light source 21 passes through lenses 22 and 23 so as to be led into an optical fiber 24 held in an optical fiber holding member 25. In this case, the optical axis of the laser light coincides with the central axis of the optical fiber 24.
An entrance end surface of the optical fiber 24 is polished so as to be perpendicular to the direction of the optical axis. Generally, reflected light of about 4% is generated in an interface between the air and a glass surface which is an end surface of the optical fiber 24 not subjected to anti-reflection processing. For this reason, the reflected light from the end surface of the optical fiber 24 passes through the lenses 23 and 22 again and returns to a laser oscillation portion of the semiconductor laser light source 21. Accordingly, there is a problem that the laser light output becomes unstable.
FIGS. 15 and 16 are views showing structures of systems for coupling semiconductor laser light sources and optical fibers to each other in order to solve the problem of such unstable laser light output. The structures have been disclosed in Patent Documents 1 and 2 respectively.
As shown in FIG. 15, in Patent Document 1, an optical fiber 24 having an entrance end surface polished obliquely with respect to the direction of the optical axis is used so that the direction of movement of reflected light from the entrance end surface of the optical fiber 24 is deflected to a direction different from the angle of incident light. Moreover, a semiconductor laser light source 21 and a lens 22 are shifted from the optical axis of the optical fiber 24 and another lens 23 nearer to the optical fiber 24 to make laser light incident obliquely on the optical fiber 24 so that the direction of movement of light refracted at the entrance end surface and made incident on the optical fiber 24 coincides with the axis of the optical fiber 24.
As shown in FIG. 16, in Patent Document 2, an optical fiber 24 having an entrance end surface polished obliquely with respect to the direction of the optical axis is used in the same manner as in FIG. 15 so that the direction of movement of reflected light from the entrance end surface of the optical fiber 24 is deflected to a direction different from the angle of incident light. Moreover, there is provided a structure that an angle of inclination is given to a semiconductor laser light source 21 itself.
Other proposals have been described in Patent Documents 3 and 4. Patent Document 3 has disclosed a method for setting an angle of inclination of the axial center of a single mode optical fiber with respect to the optical axis of a beam emitted from a light source having an elliptic near-field pattern and setting an angle of inclination of a perpendicular line dropped from the entrance end surface with respect to the axial center of the single mode optical fiber.
Patent Document 4 has disclosed a method in which: a linearly polarized beam is used as a laser beam incident on an optical fiber; the laser beam is made incident on the optical fiber so that an angle of incidence with respect to the entrance end surface of the optical fiber is a Brewster angle; and the angle of the entrance end surface of the optical fiber is set so that the laser beam is parallel to the axis of the optical fiber.
Any of the aforementioned Patent Documents 1 to 4 employs a method in which the direction of movement of reflected light from the entrance end surface of the optical fiber is deflected to a direction different from the angle of incident light.
A method different from those in Patent Documents 1 to 4 has been disclosed in Patent Document 5. That is, Patent Document 5 has disclosed a method for an optical module having a receptacle, in which a return light preventing unit is provided in part of an inner wall surface of the receptacle.
Patent Document 1: JP-B-7-119857
Patent Document 2: JP-B-6-12367
Patent Document 3: JP-A-60-191211
Patent Document 4: JP-A-5-196845
Patent Document 5: JP-A-2007-41516
The optical system described in each of Patent Documents 1 and 2 however has a disadvantage that aberration increases because marginal rays of light incident on the optical fiber are displaced largely from the optical axes of the lenses 22 and 23.
Although means for eliminating the disadvantage of Patent Documents 1 and 2 has been described in Patent Documents 3 and 4, there is no consideration about how to prevent reflected light from returning to the laser oscillation portion.
In Patent Document 5, the optical module having the receptacle is configured so that the return light preventing unit is provided in part of the inner wall surface of the receptacle. However, there is no consideration about how to prevent reflected light from returning to the laser oscillation portion.
As another unit for preventing reflected light from returning to an oscillation portion of a semiconductor laser light source in a semiconductor laser module, there is a method using an optical isolator (i.e. an element as a combination of a Faraday rotator and a polarizer) in the semiconductor laser module. Although an RIG (Rare Earth Iron Garnet) film is used as a Faraday rotator for optical communication, the RIG film cannot be used for a short-wavelength laser because the RIG film contains iron so that the RIG film does not transmit visible light with a wavelength of 1 μm or shorter unless the RIG film is considerably thin.
Therefore, TGG (Terbium Gallium Garnet) is used as a rotator for a short-wavelength region. It is however necessary to make the optical path length longer because the rotation angle per unit thickness of TGG is small. To make TGG function as an optical isolator, a large size of several cm is required. Moreover, a powerful and large magnet is required because it is necessary to apply a uniform and intensive magnetic field to TGG in order to obtain high isolation.
In a semiconductor laser module in which an optical fiber and an incident beam have to be aligned with each other in high accuracy of the order of microns, thermal expansion of constituent members is not negligible. Lowering of reliability is therefore caused by increase in size of the semiconductor laser module and expansion of the optical path length. For the aforementioned reason, the optical isolator using TGG has a problem that increase in size of the module is not avoidable.